Absolute Scale Modelling of SAXS Data From Self-Assembled Systems
Henriette Gavlshøj Mortensen · 2021
Amphiphilic molecules tend to self-assemble in solution and form particles on a nanometer scale, alone and in the presence of proteins. Small-angle X-ray scattering allows these systems to be studied in a setting close to their natural environment. By analyzing scattering data on an absolute scale, it is possible to obtain knowledge on the shape, size, cross section, stoichiometry and interactions of particles. By applying this method to two different of systems, one being proteins and surfactants, the other being phospholipid bicelles, a catalogue of algorithms for structural determination of different shapes and models was developed. Small-angle X-ray scattering was combined with fluorescence measurements, isothermal titration calorimetry, nuclear magnetic resonance measurements and other biophysical methods. This enabled the deduction of structure and crucial parameters of protein surfactant complexes made of myoglobin and α-lactalbumin with the biosurfactant rhamnolipid and α-lactalbumin and ubiquitin with the surfactant sodium dodecyl sulfate. All protein-surfactant systems studied form complexes of micelles with surfactant tails constituting the core and a shell made of surfactant head groups and partly denatured protein at saturation. Differences related to surfactant nature and protein size were observed. Phospholipid bicelle structure was studied at a varying ratio, Q, between 1,2-dimyristoyl-sn-glycero-3-phosphocholine (long-tail lipid) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (short-tail lipid). By increasing the amount of the long-tail lipid (Q increase), the structure changes from a small ellipsoidal micelle through a disc-like cylinder slice and into rectangular cuboids which grow in size as Q increases. When only the long lipid is present, multilamellar structures mixed with bilayers form. These results add a detailed perspective to the ideal bicelle model, which states that this system forms discs which grow with increasing Q. Together with an increased understanding of self-assembling systems, this thesis provides a demonstration of absolute scale modelling as a very valuable and versatile tool to deduce structural models.